The white balance device of a conventional image sensing apparatus will be explained.
An image signal output from an image sensing element is converted into a digital signal via an A/D converter. The digital signal output from the image sensing element is divided into an arbitrary number of blocks. For each block, color evaluation valuesCx=(R−B)/Y Cy=(R+B−2G)/Y Y=(R+G+B)/2are calculated (example of transformations for primary color signals).
A block where a preset white detection range to be described later contains color evaluation values (Cx and Cy) is assumed to be white. The integral values (SumR, SumG, and SumB) of color pixels of the block assumed to be white are calculated. White balance gains are calculated from these integral values:kWB—R=1.0/SumR kWB—G=1.0/SumG kWB—B=1.0/SumB 
FIG. 2 is a graph showing an example of the white detection range. White is sensed every arbitrary color temperature step under a light source ranging from a high color temperature to a low color temperature. A color evaluation value is calculated from a signal value obtained by an image sensing element, and plotted. As is represented by (a) in FIG. 2, a white determination line from a high color temperature to a low color temperature is obtained. In practice, even white slightly varies in spectral characteristics, and the line is given a small margin.
The conventional white balance detection device suffers the following problem.
For example, when a human skin is sensed by close-up under a high-color-temperature light source such as sunbeams, the color evaluation value of a white object under the sun distributes like (White in FIG. 2). The color evaluation value of the human skin distributes at almost the same position as that of the white color under a low-color-temperature light source, like (Skin in FIG. 2). For a large area of the skin color, the light source is misjudged as a lower-color-temperature light source than an actual one.
To avoid such misjudgment, the conventional white balance device is designed not to detect a low-color-temperature light source by narrowing the white detection range to a range “White” on the assumption that light is external light for a high object illuminance at high possibility.
However, the conventional white balance device cannot cope with a very high illuminance of tungsten light in a studio or the like, whose color temperature is low. For a dark object in the shade or evening, the white balance device must detect light sources from a high-color-temperature one to a low-color-temperature one. The white detection range must be widened, and misjudgment caused by a deviation to the skin color cannot be prevented.